Guardians of the Galaxy. And it seems a particularly fair comparison to me; both are quite entertaining movies, although both are in good measure re-packaged versions of earlier Marvel films.
But let's start with what Ant-Man brings to the table that's new. This is easily the most humorous entry in the canon. It features the most unlikeliest of the "unlikely heroes" yet seen in Marvel, not-entirely-reformed burglar Scott Lang. It also features the franchise's first real mentor/pupil relationship, as shrinking-tech inventor Hank Pym trains Lang to become Ant-Man. Much of the story is cast in the mold of one of my favorite film sub-genres, the "heist" movie. Not only do all of these elements stake out new Marvel territory (or push farther than they have before), they all do so quite successfully.
Another thing this film handles well (which has been hit and miss in past Marvel films) is giving the characters real problems and personality. I personally believe the two Captain America films have been the best of their respective Phases, in large measure because Steve Rogers had believable relationships that meant something to him (Peggy Carter and Bucky Barnes). Here in Ant-Man, Scott Lang is yearning to be part of his daughter's life while trying to negotiate a difficult relationship with his ex-wife and her new boyfriend. At the same time, Hank Pym is mourning the loss of his own wife, and trying to make amends for how that loss destroyed his relationship with his daughter. In short, there are personal stakes underpinning the superheroics that an audience can relate to.
Yet with all of these compelling and original threads in the weave, Marvel can't help but include the familiar. Much as Guardians of the Galaxy was a re-skin of The Avengers, Ant-Man is a re-skin of the original Iron Man. A brilliant inventor has lost control of the company he created, but has retained the secret of a powerful technology. The ambitious new man running the company is crazy for one-dimensional reasons that won't bear scrutiny, and is trying to unlock the secret of the new invention himself for profit and evil. It all builds to the creation of a supervillain just 30 minutes before the movie ends, who has a big fight with the hero.
Yes, I was disappointed in how familiar Ant-Man ended up, after so much potential for difference that it started with. Still, I liked it overall, and that has a lot to do with the cast. Paul Rudd makes a very accessible protagonist that's easy to root for -- a skill he's showcased in many non-superpowered movies before. Michael Douglas is a fun mentor with just the right amount of a mean streak. Evangeline Lilly plays great against both of them, funny and dramatic in turn. Michael Peña nearly steals the show as Scott Lang's hilarious former cellmate. And, not getting too specific, the appearances of other people from the Marvel Universe are quite effective and welcome here (including the obligatory Stan Lee cameo that, for once, I actually quite enjoyed).
In fact, to whatever degree Ant-Man does compare to Iron Man in my mind, Ant-Man comes off better on every level. It's a lot of fun, and I give it a B+.